Administrative Distance

To send message, there can be multiple paths from source to destination. The routers learn about these paths from routing protocols. If a route is configured with multiple routing protocols and every router is giving different paths to reach the same destination, then the router will use a number associated with those routing protocols. The path given by that routing protocol who will have least number associate with it will be selected as the best path to reach the destination. This number is known as the administrative distance.
Administrative distance shows the reliability/trustworthiness of a routing protocol, Smaller the AD (administrative distance), the more reliable the protocol.

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